1. Field of the Invention
The present invention relates to a gas sensor.
2. Description of the Related Art
A conventionally known gas sensor detects a specific gas concentration such as NOx in a gas to be measured or a measurement-object gas such as an exhaust gas of an automobile. For example, JP 3798412B and JP 2011-102797A describe gas sensors, each including a sensor element of a long plate-like shape formed by stacking a plurality of airtight oxygen ion-conductive solid electrolyte layers.
FIG. 6 is a sectional schematic diagram schematically illustrating one exemplary configuration of a prior art gas sensor 300. As illustrated, this gas sensor 300 includes a sensor element 307. The sensor element 307 is provided as an element of stacked structure by stacking oxygen ion-conductive dense solid electrolyte layers 301 to 306. In this sensor element 307, a measurement-object gas flowing portion in which measurement-object gas is introduced is formed between a lower surface of the solid electrolyte layer 306 and an upper surface of the solid electrolyte layer 304. The measurement-object gas flowing portion includes a gas inlet 310 and first to third internal cavities 320, 340 and 361. A measurement electrode 344 is formed on a lower surface of the third internal cavity 361 (upper surface of the solid electrolyte layer 304). An outer pump electrode 323 is formed on an upper surface of the solid electrolyte layer 306. A reference gas introducing space 343 which a reference gas used as a standard for detection of a specific gas concentration in the measurement-object gas is introduced in is formed between an upper surface of the solid electrolyte layer 303 and a lower surface of the solid electrolyte layer 305. A reference electrode 342 is formed on the upper surface of the solid electrolyte layer 303 facing the reference gas introducing space 343. The reference electrode 342 is covered by a reference gas introducing layer 348 made of a porous material, and the reference gas is introduced from the reference gas introducing space 343 through the reference gas introducing layer 348 into the reference electrode 342. In this gas sensor 300, when the measurement-object gas is introduced into the third internal cavity 361 of the measurement-object gas flowing portion, an electromotive force Va is generated between the measurement electrode 344 and the reference electrode 342. Oxygen is pumped in or pumped out via the outer pump electrode 323 and the measurement electrode 344, based on this electromotive force Va. The gas sensor 300 detects the specific gas concentration in the measurement-object gas, based on an electric current Ip2 during pump-in or pump-out.